Sulfur loss from subducted altered oceanic crust and implications for mantle oxidation-猫眼课题宝

DOI：

10.7185/geochemlet.2011

发表时间：

2020-04

期刊：

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其他

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作者：

J. Walters;A. Cruz‐Uribe;H. Marschall

doi: 10.7185 / geochemlet.2011氧逸度（fO2）是地幔物理的一个控制因素；然而，与趋同边缘相关的fO2空间和长期变化的驱动机制存在很大争议。我们提出了新的热力学模型和岩石学观测来预测氧化硫物质是在蚀变海洋地壳俯冲过程中产生的。俯冲板块的硫损失是原岩Fe3+/ΣFe比值和俯冲带热结构的函数，预测在冷俯冲带氧化板块硫通量升高。我们还预测了含硫流体的双峰释放，还原硫的浅层通量低，随后是硫酸盐和亚硫酸盐物种的深层通量增强，与氧化弧岩浆和伴生的铜斑岩矿床一致。我们的模型预测的跨活动边缘和活动边缘的可变SOx释放可能会引入上地幔的fO2非均质性。收稿日期2019年9月27日收稿日期2020年2月26日收稿日期2020年4月2日发布日期2.美国缅因大学地球与气候科学学院，5790 Bryand全球科学中心，美国缅因州奥罗诺044692 .德国<s:1>地理科学研究所，Goethe-Universität法兰克福，Altenhöferallee 1, 60438作者简介俯冲作用可能通过引入水合氧化蚀变海洋地壳影响地幔和幔源岩浆的氧逸度（fO2）（AOC, Fig. 1; Evans, 2012）。地幔fO2通过矿物学变化调节地幔流变学和密度；例如，名义上无水矿物的H2O含量是fO2的函数（McCammon et al., 2004）。因此，板块构造引起的地幔fO2的长期变化可能驱动地幔环流的变化（Mackwell, 2008）。然而，俯冲板块中氧化还原敏感元素与俯冲带地幔fO2之间的联系尚不明确。越来越多的证据表明，地幔fO2的演化是对氧化板块组分转移的响应。例如，来自墨西哥的橄榄岩捕虏体记录了石英-铁橄榄石-磁铁矿（QFM）缓冲带上方1.5-2.4 log单位的fO2条件（布拉特和卡迈克尔，1998），而洋中脊橄榄岩与QFM重叠（Birner等人，2018）。同样，弧岩浆相对于海中脊玄武岩被氧化（MORB； Kelley和Cottrell， 2009年，2012年；Cottrell和Kelley， 2011年；Brounce等人，2014年）。弧岩浆的Fe3+/ΣFe比值与陆块向岩浆源添加物质的地球化学指标如Ba/La比值正相关（Kelley and Cottrell, 2009）；因此，沿辐合边缘抬升的地幔fO2在空间和化学上都与俯冲板块有联系。这种联系可能需要从板块岩性中转移氧化还原敏感元素，这些元素相对于地幔橄榄岩被氧化。早期的研究假设引入板Fe3+（例如，Lecuyer和Ricard， 1999）；然而，Fe3+在含水流体中的溶解度很低（Mungall， 2002年）。相反，挥发物可能起着更重要的作用（Evans, 2012）。其中，只有氢、碳和硫的丰度足以影响地幔的氧化还原态。硫和碳是流体流动的，在氧化态表现出8个电子范围，以1012 mol/yr的全球速率俯冲，可能是氧化态转移的重要载体（Evans, 2012）。重要的工作集中在俯冲过程中的脱碳，而硫的损失仍然很少被探索。尽管与C-CO2的转变相比，从s2到SO4的转变发生在更氧化的条件下（图S-1）。氧化碳在正常上地幔P-T-fO2下稳定；因此，板块衍生的CO2通量不能在地幔楔中引发铁氧化反应（见补充资料S-2）。相比之下，氧化硫的通量可将次弧地幔的log（fO2）提高到~QFM + 2，与次弧地幔捕虏体中通常观察到的范围一致（例如，Blatter和Carmichael， 1998）。因此，硫仍然是俯冲带中最强大的氧化剂。最近的研究表明，平板流体中的还原型（H2S, HS-）或氧化型（SO4）硫有可能降低或氧化亚弧地幔（例如Evans）

doi: 10.7185/geochemlet.2011 Oxygen fugacity ( fO2) is a controlling factor of the physics of Earth’s mantle; however, the mechanisms driving spatial and secular changes in fO2 associated with convergent margins are highly debated. We present new thermodynamic models and petrographic observations to predict that oxidised sulfur species are produced during the subduction of altered oceanic crust. Sulfur loss from the subducting slab is a function of the protolith Fe3+/ΣFe ratio and subduction zone thermal structure, with elevated sulfur fluxes predicted for oxidised slabs in cold subduction zones. We also predict bi-modal release of sulfur-bearing fluids, with a low volume shallow flux of reduced sulfur followed by an enhanced deep flux of sulfate and sulfite species, consistent with oxidised arc magmas and associated copper porphyry deposits. The variable SOx release predicted by our models both across and among active margins may introduce fO2 heterogeneity to the upper mantle. Received 27 September 2019 | Accepted 26 February 2020 | Published 2 April 2020 1. School of Earth and Climate Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, Maine 04469, USA 2. Institut für Geowissenschaften, Goethe-Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, Germany 3. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA * Corresponding author (email: jesse.walters@maine.edu) Introduction Subduction may influence the oxygen fugacity ( fO2) of the mantle and mantle-derived magmas through the introduction of hydrated and oxidised altered oceanic crust (AOC, Fig. 1; Evans, 2012). Mantle fO2 can regulate mantle rheology and density through changes in mineralogy; for example, the H2O content of nominally anhydrous minerals is a function of fO2 (McCammon et al., 2004). As a result, secular changes in mantle fO2 induced by plate tectonics may drive variations in mantle circulation (Mackwell, 2008). However, the link between redox sensitive elements in subducting slabs and mantle fO2 at subduction zones remains elusive. Mounting evidence suggests that mantle fO2 evolves in response to a transfer of oxidised slab components. For example, peridotite xenoliths from Mexico record fO2 conditions 1.5–2.4 log units above the quartz-fayalite-magnetite (QFM) buffer (Blatter and Carmichael, 1998), whereas mid-ocean ridge peridotite overlaps with QFM (Birner et al., 2018). Similarly, arc magmas are oxidised relative to mid-ocean ridge basalts (MORB; Kelley and Cottrell, 2009, 2012; Cottrell and Kelley, 2011; Brounce et al., 2014). The Fe3+/ΣFe ratios of arc magmas positively correlate with geochemical indicators of material addition from the slab to the magma sources, such as Ba/La ratios (Kelley and Cottrell, 2009); therefore, elevated mantle fO2 along convergent margins is both spatially and chemically linked to the subducting slab. Such a link may require the transfer of redox sensitive elements from slab lithologies, which are oxidised relative to mantle peridotite. Early studies hypothesised the introduction of slab Fe3+ (e.g., Lecuyer and Ricard, 1999); however, the solubility of Fe3+ in hydrous fluids is low (Mungall, 2002). Instead, volatiles likely play a more important role (Evans, 2012). Of these, only H, C, and S potentially occur in sufficient abundance to influence the redox state of the mantle. Sulfur and carbon are fluid mobile, exhibit an eight electron range in oxidation states, are subducted at global rates on the order of 1012 mol/yr, and may act as important vectors for transferring oxidation state (Evans, 2012). Significant work has focused on decarbonation during subduction, whereas sulfur loss remains less explored. This is despite the fact that the transition from S2to SO4 occurs at more oxidising conditions relative to the C-CO2 transition (Fig. S-1). Oxidised carbon is stable at normal upper mantle P–T–fO2; therefore, slab-derived CO2 fluxes are unable to initiate Fe oxidising reactions in the mantle wedge (see Supplementary Information S-2). In contrast, a flux of oxidised sulfur may raise log( fO2) of the subarc mantle to ~QFM + 2, consistent with the range commonly observed in subarc mantle xenoliths (e.g., Blatter and Carmichael, 1998). Sulfur thus remains the most powerful oxidising agent in subduction zones. Recent studies have favoured either reduced (H2S, HS-) or oxidised (SO4) sulfur species in slab fluids, with the potential to reduce or oxidise the subarc mantle (e.g., Evans

Sulfur loss from subducted altered oceanic crust and implications for mantle oxidation

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